Chemical vapor deposition (CVD) systems or reactors are well known and widely used to deposit or grow thin films of various compositions upon surfaces of substrates. For example, CVD systems are commonly used to deposit dielectric, passivation and dopant layers upon semiconductor wafers. CVD systems operate by introducing a process gas or chemical vapor into a deposition chamber in which a substrate to be processed has been placed. The gaseous source chemicals pass over the substrate, are adsorbed and react on the surface of the substrate to deposit the film. Various inert carrier gases may also be used to carry a solid or liquid source into the deposition chamber in a vapor form. Typically, the substrate is heated from 200 to 900.degree. C. to initiate the reaction.
One type of CVD system used in semiconductor manufacturing is an atmospheric pressure chemical vapor deposition system (hereinafter APCVD system). APCVD systems are described in, for example, U.S. Pat. No. 4,834,020, to Bartholomew et al., which is incorporated herein by reference. In an APCVD system, a deposition chamber is maintained at atmospheric pressure while gaseous source chemicals are introduced to react and deposit a film on the substrate. A common embodiment of the APCVD system uses a belt or conveyor to move the substrates through the deposition chamber during the deposition process. Because this design allows uninterrupted processing of substrates, and because APCVD systems generally provide a higher rate of film growth than, for example, low pressure CVD systems in which the chamber must be evacuated prior to each deposition process, belt-driven APCVD systems typically provide a much greater substrate throughput.
Referring to FIG. 1, a conventional belt-driven APCVD system 20 typically includes an endless wire belt 25 for transporting substrates 30 at elevated temperatures through a process muffle 35 having a series of chambers 40, each chamber having a process gas injector 45 for depositing a layer (not shown) on the substrates. The substrates 30 on the belt 25 are heated by heaters 50 below a floor 55 of the process muffle 35. To provide a film having a uniform thickness across a substrate 30, and from substrate to substrate, the substrates must be heated uniformly during the deposition process. Thus, there must be good thermal contact between the substrates 30 and the belt 25, and between the belt and the muffle floor 55.
A shortcoming of conventional APCVD systems is that deposition occurs not only on the substrates, but also on components of the system itself. One problem caused by deposits on the APCVD system components is that they tend to chip or flake, generating particles that can contaminate the substrates. To ensure uniform and repeatable processing of the substrates, these deposits must be periodically removed from the system components. In particular, to uniformly heat the substrates in order to obtain consistently uniform films, the deposits must be removed from the belt and the muffle floor to provide flat contact surfaces between the substrates and the belt and between the belt and the muffle floor. Furthermore, the floor of the muffle directly below each deposition chamber typically includes a number of perforations through which a purge gas is introduced to inhibit film deposition on the back sides of processed substrates. If these perforations become clogged over time by CVD deposits, there is an insufficient flow of purge gas through the perforations and backside deposition on the substrates can occur.
Thus, there is a need for an APCVD system that reduces the generation of particles and reduces the accumulation of deposits on the APCVD system components generally and in particular on the belt and the muffle floor.
An additional liability of conventional APCVD systems constructed from chromium-containing alloys including stainless steels and many nickel-based alloys is associated with the tendency of these alloys to form protective chromium oxide surface layers that give these alloys their desirable oxidation resistance. The chromium oxide surface layer that forms on these alloys after extended service is known to generate gaseous chromium-containing compounds when employed in APCVD systems; these compounds condense on processed substrates resulting in chromium contamination.
Several approaches have been attempted to reduce the build up of deposits on the belt. One generally known approach, shown in FIG. 1, uses a belt cleaning mechanism 60 to continually clean the belt 25 during operation of the APCVD system. The belt cleaning mechanism 60 has an etch muffle 65 below the process muffle 35. After exiting the chambers 30 and the process muffle 35 the belt 25 enters the etch muffle 65 in which gaseous hydrous hydrogen fluoride (HF) in a nitrogen carrier gas (typically an azeotropic concentration) is passed through the belt, to react with and etch deposits on the belt. The belt 25 is then passed through an ultrasonic bath 70 of flowing deionized water, in which agitation removes etch products and particles from the belt. Finally, before reentering the process muffle 35, the belt 25 passes through an air knife 75 and an infrared dryer 80 to dry the belt.
Although the above approach keeps the belt relatively free of built-up deposits, the muffle floor 55 eventually accumulates deposits to such a degree that nonuniform heating of the substrate occurs, causing poor film uniformity on substrates, and/or the purge perforations to become clogged. When this occurs, the muffle floor must be cleaned, and an HF cleaning procedure, commonly referred to as muffle etch, has been developed for this purpose. This procedure requires cooling the process muffle to near-room temperatures and dismantling the system to gain access to the muffle floor region. Typically, the procedure requires the replacement of the injector assemblies by HF dispensing tanks. These tanks expose the muffle floor to aqueous or gaseous hydrous HF that removes the accumulated material. The frequency of these muffle etches is dependent on several factors including the type and amount of chemicals used and the temperature of the deposition chamber. Typically, for conventional APCVD systems used to process semiconductor wafers the mean time between muffle etches (MTBME) is on the order of every one to two hundred hours. This need for frequent, invasive and time consuming cleaning limits the otherwise excellent substrate throughput capabilities of APCVD systems.
Accordingly, there is a need for an APCVD system, and a method of operating the system, that reduces the formation of deposits on the system components in general, and in particular on the belt and the muffle floor. There is also a need for an APCVD system that increases the MTBME, thereby increasing substrate throughput. There is a further need for a method of reducing the generation of gaseous metal-containing compounds from the interior surfaces of the metal components in APCVD systems.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.